Segmented backlight structure

ABSTRACT

An LED backlight system having a plurality of backlight segments including an integral light waveguide, each backlight segment supporting a sidelight emitting LED. A light guide having a rectangular shape, a flat top surface, and a curved lower surface defines a cavity having a top and sidewalls, with the sidelight emitting LED positioned in the cavity. At least one of a reflective layer and a top out-coupling structure can be positioned between the top of the cavity and the sidelight emitting LED.

FIELD OF INVENTION

This invention relates to the field of light emitting devices, and inparticular to a pixelated light emitting backlight structure for highdynamic range display devices.

BACKGROUND

Mobile, tablet, laptop, or larger electronic displays ideally have alarge contrast between dark and light parts of the image. Since a humaneye has a very high dynamic range (HDR) of 14 orders of magnitude,displays that support high contrast range allow for more faithful imagesto be rendered. High dynamic range displays should have high contrast, avery black state and high peak brightness, and allow for precise controlof light levels. For example, an organic LED (OLED) based display cantypically have perfect black (no light), offering a high contrast thatsupports HDR viewing. In comparison, liquid crystal displays (LCD) arenot able to support perfect black due to light leakage from thebacklight through the optical system. However, maximum light output forLCDs is generally higher than for OLED displays, allowing for extensionof the dynamic range at the brighter end. If black values can besignificantly reduced, LCD displays can match or exceed the dynamicrange of OLEDS.

One recognized way of reducing black values in LCD displays is to switchoff the backlight (locally) to enhance the contrast of an image. Thesmaller the area of the backlight that can be switched off, the betterthe contrast resolution. This technique is often applied in LCD based TVsets and is commonly called full array local dimming (FALD).

Unfortunately, conventional FALD techniques do not work on smallerdisplays provided for laptops, tablets, and mobile devices, primarilydue to thinness (typically less than 0.4 mm) of the supportingbacklight. LEDs supporting FALD are embedded in a light guide andnormally emit light from the top. With a very thin backlight, the LEDwould be clearly visible through the light guide layer and the rest ofthe optical system (brightness enhancement and diffuser foils).Replacement of top-lighting LED sources with sidelight-emitting LEDshaving virtually no emission from the top of the package is notfeasible, since this would create a “black hole” right above LED, ratherthan a uniform illuminance.

SUMMARY

In accordance with embodiments of the invention, an LED backlight systemincludes a plurality of backlight segments including an integral lightwaveguide, each backlight segment supporting a sidelight emitting LED. Alight guide having a rectangular shape, a flat top surface, and a curvedlower surface defines a cavity having a top and sidewalls, with thesidelight emitting LED positioned in the cavity. At least one of areflective layer and a top out-coupling structure can be positionedbetween the top of the cavity and the sidelight emitting LED.

In some embodiments the backlight segments are separated from each otherby an optical barrier which can include a reflective coating or lightabsorbing layer. Each of the plurality of backlight segments can bepositioned adjacent to at least one other backlight segment on a printedcircuit board substrate.

A sidewall out-coupling structure can be positioned between thesidewalls of the cavity and the sidelight emitting LED. In someembodiments only the top out-coupling structure is positioned betweenthe top of the cavity and the sidelight emitting LED, while in others anadditional reflective layer is formed on the top out-coupling structure.

The plurality of backlight segments can be positioned adjacent to eachother on a printed circuit board substrate and can be similarly sized toallow for modular layouts. Each sidelight emitting LED can be centrallypositioned in the respective backlight segment.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present disclosureare described with reference to the following figures, wherein likereference numerals refer to like parts throughout the various figuresunless otherwise specified.

FIG. 1 illustrates one embodiment of an LED display system includingindividually controlled segmented backlight;

FIG. 2A illustrates one embodiment of an LED backlight with a lightwaveguide having a cavity into which an LED is fitted;

FIG. 2B is a simulation result illustrating near uniform light outputfor an LED backlight such as shown in FIG. 2A;

FIG. 3A illustrates one embodiment of an LED backlight having a topout-coupling structure positioned between the top of the cavity and ansidelight emitting LED;

FIG. 3B is a simulation result illustrating near uniform light outputfor an LED backlight such as shown in FIG. 3A;

FIG. 4A illustrates one embodiment of an LED backlight with a curved andtapered light waveguide having a cavity into which an LED is fitted;

FIG. 4B is a perspective view curved and tapered light waveguide such asshown in FIG. 4A;

FIG. 5A illustrates one embodiment of an LED backlight with a curved andrectangular edged light waveguide having a cavity into which an LED isfitted;

FIG. 5B is a perspective view of a curved and rectangular edged lightwaveguide such as shown in FIG. 5A;

FIG. 6A illustrates relative position of a cube shaped LED in a squarecavity in the light waveguide;

FIG. 6B illustrates relative position of a cube shaped LED in a squarecavity in the light waveguide, with the light waveguide supportingprisms for light in-coupling;

FIG. 7A is a simulation result illustrating near uniform light outputfor an LED backlight such as shown in FIG. 6B; and

FIG. 7B is a simulation result illustrating non-uniform light output foran LED backlight without prisms for light in-coupling.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, for purposes of explanation rather thanlimitation, specific details are set forth such as the particulararchitecture, interfaces, techniques, etc., in order to provide athorough understanding of the concepts of the invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced in other embodiments, which depart from these specificdetails. In like manner, the text of this description is directed to theexample embodiments as illustrated in the Figures and is not intended tolimit the claimed invention beyond the limits expressly included in theclaims. For purposes of simplicity and clarity, detailed descriptions ofwell-known devices, circuits, and methods are omitted so as not toobscure the description of the present invention with unnecessarydetail.

FIG. 1 illustrates one embodiment of an LED backlight display system 100including an individually controlled segmented backlight 102. A printedcircuit board 110 supports each light waveguide segment 120 and connectsLED(s) to a backlight controller 150. The LEDs 140 are positioned withincavities 130 defined within the light waveguide segments 120. Nearuniformly distributed light 123 of a controllable and distinct intensitycan be emitted from a top surface 124 of each backlight segment 102 topass through an LCD pixel display element suitable for mobile, tablet,laptop, or other devices. In some embodiments, full array local dimming(FALD) is supported even though thickness of the light waveguide segment120 can be significantly less than 1 mm, and in certain embodiments,between about 0.3 to 0.5 mm in thickness. The LED backlight displaysystem 100 such as described can also support a homogeneity of 90%,either defined as a contrast between light and dark in specific regionof interest, or as the standard deviation in illuminance divided by theaverage illuminance in the region of interest.

In some embodiments, single or multiple LEDS can be associated with eachlight waveguide segment 120. In other embodiments, cubic or rectangularLEDs with strong sidelight emission on all four sides and limited topand bottom emission can be used. Top emission can be further limited byscattering elements or applied reflective or light absorbing coatings.Semiconductor materials capable of forming high-brightness LEDs caninclude, but are not limited to, indium gallium nitride, aluminumgallium nitride, aluminum gallium indium nitride, and other suitableGroup III-V semiconductors, particularly binary, ternary, and quaternaryalloys of gallium, aluminum, indium, and nitrogen, also referred to asIII-nitride materials. As will be appreciated, various othermodifications to the LED architecture and materials are possible. LEDsegments can be overlain with wavelength converting materials such asphosphors, quantum dots, or dyes. Multiple types and thicknesses ofphosphors can be used. An LED combined with one or more wavelengthconverting materials may be used to create white light or monochromaticlight of other colors. In some embodiments, microlenses or other primaryor secondary optical elements (including reflectors, scatteringelements, or absorbers) may be attached to each LED or associatedphosphor. Protective layers, transparent layers, thermal layers, orother packaging structures can be used as needed.

Each LED is positioned with a cavity 130 defined within a lightwaveguide segments 120. In some embodiments each light waveguide segment120 has rectangular or square boundaries and can be constructed to havea common size to simplify modular increase or decrease in the number andlayout of the segments 120. In some embodiments the light waveguidesegments are 5×5 mm slab with a thickness of less than 0.4 mm. Typicalmaterial include poly(methyl methacrylate) (PMMA), other acrylics, orother optical grade plastic.

The cavity 130 can be generally cubic, rectangular, truncated pyramidal,circular, elliptical, or other suitably defined shape able toaccommodate an LED. In some embodiments, a small gap is created betweenthe top of the LED and the top of the cavity. A gap can also be formedbetween the sides of the LED and the cavity sidewalls Advantageously,these gaps reduce required tolerances for placement of the LED withinthe cavity, simplifying manufacture and assembly. Top emission can bereduced using specular or diffuse reflective coatings. In-coupling andout-coupling of light from the light waveguide segments 120 can beimproved by use of 3-dimensional scattering structures, defined ormolded grooves, pyramids, or notches, as well as printed white dotpatterns. These structures or patterns can be defined on the cavitysidewalls or top of the cavity. In some embodiments, reflective orscattering layers can be formed over such light in and out-couplingstructures. In particular embodiments, at least one of a reflectivelayer and a top out-coupling structure are positioned between the top ofthe cavity and the LED.

In other embodiments, an edge of the light guide segment 120 can includeoptical absorbers or reflectors to reduce light crosstalk betweenadjacent light guide segments 120. Deflecting light through the topsurface can be enabled by suitable out-coupling structures defined orpositioned within the light guide segments. These can be 3-dimensionalscattering structures, defined or molded grooves, pyramids, or notches,as well as printed white dot patterns. Homogeneity can also be improvedby use of scattering, homogenizing, or diffuser foils or filmspositioned over the top surface 124. In some embodiments, brightnessenhancement foils can also be included to improve peak luminance. Suchfoils or films can be discrete, covering a single light waveguidesegment 120, or can cover two or more light waveguide segments 120.Advantageously, for those embodiments in which a foil or film iscoextensive with the backlight, a low level of lateral light diffusioncan help smooth over visual seams and reduce any visual gradientsbetween light waveguide segments 120.

FIG. 2A illustrates (in cross section) one embodiment of an LEDbacklight 200 showing a single integrally formed light waveguide segment220 supported on a printed circuit board 210. The light waveguidesegment 220 has a cavity 230 defined therein into which a sidelightemitting, generally cubical or rectangular solid shaped LED 240 isfitted. A top of the cavity 232 is coated with a reflector 234. Lightin-couplings structures can be formed or defined in cavity sidewalls236, while light out-coupling structures 260 can be formed on or nearthe bottom surface 231 of the light waveguide segment 220. In effect,side emitted light from the LED 240 enters the light waveguide segment220 and is redirected to provide a nearly homogeneous backlight 223.Light transfer to any adjacent light waveguide segments is limited by areflective light barrier 238 covering edges of the light waveguidesegment 220.

FIG. 2B is a simulation result graph 201 illustrating near uniform lightoutput for an LED backlight such as shown in FIG. 2A. As compared toother disclosed embodiments herein, a recognizable central brightnesspeak is still present when a top reflector is used alone.

FIG. 3A illustrates one embodiment of an LED backlight having a topout-coupling structure positioned between the top of the cavity and asidelight emitting LED. Similar to the structure discussed with respectto FIG. 2A, FIG. 3A has an LED backlight 300 including a singleintegrally formed light waveguide segment 320 supported on a printedcircuit board 310. The light waveguide segment 320 has a cavity 330defined therein into which a sidelight emitting, generally cubical orrectangular solid shaped LED 340 is fitted. A top of the cavity 332 isfirst coated with out-coupling structures 337 that can include formed ordefined structures or scattering layers. A reflector 334 is formed onthe out-coupling structures 337. Light in-coupling structures can beformed or defined in cavity sidewalls 336, while light out-couplingstructures 360 can be formed on or near the bottom surface 331 of thelight waveguide segment 320. In effect, side emitted light from the LED340 enters the light waveguide segment 320 and is redirected to providea nearly homogeneous backlight 323. Light transfer to any adjacent lightwaveguide segments is limited by a reflective light barrier 338 coveringedges of the light waveguide segment 320.

FIG. 3B is a simulation result illustrating near uniform light outputfor an LED backlight such as shown in FIG. 3A. As can be seen, use ofcombined reflector 336 and out-coupling structures 337 results inimproved light homogeneity and a greatly reduced central brightness peakas compared to structures such as illustrated in FIGS. 2A and 2B.

FIG. 4A illustrates one embodiment of an LED backlight with a curved andtapered light waveguide having a cavity into which an LED is fitted. Asillustrated, FIG. 4A has an LED backlight 400 including a singleintegrally formed light waveguide segment 420 supported on a printedcircuit board 410. The curved and tapered light waveguide segment 420has a cavity 430 defined therein into which a sidelight emitting,generally cubical or rectangular solid shaped LED 440 is fitted. A topof the cavity 432 is first coated with out-coupling structures 437 thatcan include formed or defined structures or scattering layers. Areflector 434 is formed on the out-coupling structures 437. Lightin-couplings structures can be formed or defined in cavity sidewalls436. Light in-couplings structures can be formed or defined in cavitysidewalls 436, while light out-coupling structures (not shown) can beformed on or near the bottom surface 431 of the light waveguide segment420. Because of the curvature of the light guide and use of taperededges, little or no light is emitted from the edge, reducing oreliminating the need for edge attached light absorbers or reflectors. Inpractice, edges will have some thickness, but can generally bemanufactured to have a thickness or edge height of less than 300 micronsand can have an edge height between 50 and 200 microns, with 100 micronsbeing typical.

FIG. 4B is a perspective view of a curved and tapered light waveguide430 such as shown in FIG. 4A. As illustrated, a generally square orrectangular waveguide 430 is defined in part by four triangular shapedfaces. An LED can be mounted in a cavity defined near the center of thewaveguide 430.

FIG. 5A illustrates one embodiment of an LED backlight with a curved andrectangular edged light waveguide having a cavity into which an LED isfitted. As illustrated, FIG. 5A has an LED backlight 500 including asingle integrally formed light waveguide segment 520 that defines acurve, has an edge, and is supported on a printed circuit board 510. Thecurved and tapered light waveguide segment 520 has a cavity 530 definedtherein into which a sidelight emitting, generally cubical orrectangular solid shaped LED 540 is fitted. A top of the cavity 532 isfirst coated with out-coupling structures 537 that can include formed ordefined structures or scattering layers. A reflector 534 is formed onthe out-coupling structures 537. Light in-couplings structures can beformed or defined in cavity sidewalls 536. Light in-couplings structurescan be formed or defined in cavity sidewalls 536, while lightout-coupling structures (not shown) can be formed on or near the bottomsurface 531 of the light waveguide segment 520. An edge 535 of the lightguide segment 520 can include optical absorbers or reflectors to reducelight crosstalk between adjacent light guide segments.

FIG. 5B is a perspective view of a curved and rectangular edged lightwaveguide 530 such as shown in FIG. 5A. As illustrated, a generallysquare or rectangular waveguide 530 is defined by a smooth surfaceterminating in edges that have a lesser height at corners as compared tocenters of the respective edges. An LED can be mounted in a cavitydefined near the center of the waveguide 530.

FIG. 6A illustrates relative position of a cube shaped LED 640 in asquare cavity 630 in a light waveguide 620. In this embodiment, the LEDis rotated with respect to the cavity 630 so that edges of the LED 640face corners of square cavity 630, while edges of the cavity 630 facecorners of the LED 640.

FIG. 6B illustrates relative position of a cube shaped LED 640 in asquare cavity 631 in the light waveguide 621, with the light waveguide621 supporting sidewall prisms 633 for improved light in-coupling. Thesidewall prisms 633 effectively define a Fresnel representation of therotated cavity of FIG. 6A with the advantage of improved light spreadingfunctionality.

FIG. 7A is a simulation result graph 700 illustrating near uniform lightoutput for an LED backlight such as shown in FIG. 6B.

FIG. 7B is a simulation result graph 710 illustrating non-uniform lightoutput for an LED backlight without sidewall prisms 633 for lightin-coupling.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

What is claimed is:
 1. A LED backlight system, comprising: a pluralityof backlight segments, each backlight segment supporting a sidelightemitting LED; and a light guide segment included in each backlightsegment, with the light guide having a rectangular shape, a flat topsurface, and a curved lower surface, the light guide further defining acavity having a top and sidewalls, with the sidelight emitting LEDpositionable in the cavity, wherein the backlight segments are separatedfrom each other by an optical barrier.
 2. The LED backlight system ofclaim 1, wherein the optical barrier separating the backlight segmentsfrom each other is defined in part by a tapered edge of the light guidesegment.
 3. The LED backlight system of claim 1, wherein the opticalbarrier separating the backlight segments from each other is at leastone of a reflective and absorptive coating.
 4. The LED backlight systemof claim 1, wherein a sidewall in-coupling structure is positionedbetween the sidewalls of the cavity and the sidelight emitting LED. 5.The LED backlight system of claim 1, wherein at least one of areflective layer and a top out-coupling structure are positioned betweenthe top of the cavity in the light guide and the sidelight emitting LED.6. The LED backlight system of claim 1, wherein each sidelight emittingLED is centrally positioned in the respective backlight segment.
 7. TheLED backlight system of claim 1, wherein the curved lower surface of thelight guide includes four triangular shaped faces.
 8. The LED backlightsystem of claim 1, wherein the light guide includes a smooth lowersurface terminating in edges that have a lesser height at corners ascompared to centers of respective edges.